Method of making rubber-bound fibrous products and the like



atent 2,843,479 Patented July 15, 1958 15 Claims. ((li. 92-2l) This invention relates to a method of depositing a binder onto filler particles and is more particularly di rected to the production of a Water-laid felted fibrous sheet from a slurry of fibers and a dispersion of rubber, such as a synthetic rubber latex, or other binder which may be dispersed in Water and subsequently precipitated onto the fibers.

Felt and similar paper products are now produced by the socalled beater saturation process, wherein a suspension of fibers in water has added thereto a binder in dispersion form and the binder is deposited onto the fibers by agitation of the material, preferably with the addition of alum or other precipitating agent to the slurry to cause the binder to be deposited upon the fibers.

With certain fibrous products such as Wool, unbleached kraft stock, and other wood pulp furnishes having little or no refining treatment, conventional beater saturation techniques have been found impracticable, for the depositions of binder have been in the form of relatively large flocculated lumps distributed unevenly throughout the fiber mass.

An object of the present invention is to provide a process which will provide for the uniform deposit of a dispersed binder onto filler particles suspended in an aqueous slurry.

Another object is to provide a process for the preparation of a Water-laid sheet from fibers having a binder deposited uniformly onto the fibers from an aqueous dispersion.

A further object of the invention is to provide a process by which a uniform deposition of a dispersed binder may be obtained on wool fibers suspended in an aqueous slurry.

An additional object is to provide a method of depositing a binder in dispersed form uniformly upon fibers such as unrefined wood fibers which normally are difficult to treat in accordance with conventional beater saturation processes.

Other objects of the invention will be apparent from consideration of the following description of certain embodiments of the invention.

According to the present invention, the selected particles, such as fibers or the like, are first sensitized for the deposit thereon of a dispersed binder. This step is preferably effected by agitating the fibers in an aqueous medium having dissolved therein a limited quantity of aluminurn sulphate or its equivalent. Thereafter, the sensitized fibers are treated in the slurry with a sequestering agent which removes any excess aluminum ions which may be present. This is accomplished by adding to the slurry of sensitized fibers a limited quantity of sodium citrate or its equivalent. Upon completion of this treatment, there is added to the slurry of sensitized and treated fibers a dispersion of binder, such as a synthetic rubber latex or its equivalent; and the dispersion is caused to be deposited upon the fibers by adding to the slurry a precipitation agent for the dispersion, such as a mixture of alum and acetic acid or its equivalent, and agitating the slurry. Where the process is carried out on paper-making equipment and the white water is recirculated, it is preferred to use as the precipitation agent a material which will not affect the subsequent sensitizing of additional fibers suspended in the recirculated white water.

The following examples will illustrate certain embodiments of the invention:

Example I Parts by weight Kraft fiber 200.0 Cotton fiber 200.0 Cork granules 200.0 Alum 15.0 Sodium citrate a 10.0 Antioxidant (47% solids) (dry weight) u 1.7 Carboxy methylcellulose 5.1 Butadiene-styrene rubber latex (38% solids) (dry weight) 300.0

Water 72,0000 Aium-acetic acid precipitant solution 200.0

In the preparation of a sheet in accordance with this example, the fibers and cork granules are added to the water in a conventional'tile chest or the like provided with an agitator. The kraft fibers in this example may be northern softwood 'kraft, unbleached and having a permanganate number of about 15 or above. The cotton may be unbleached chemical cotton. The cork granules may be of a size known as 60 to dust, i. e., substantially all particles will pass through a 60 mesh per inch United States standardsieve, and the particles will range from such maximum size to dustlike particles which will pass through a 200 mesh sieve. The cork particles are preferably waterlogged prior to addition to the slurry to avoid their floating on the surface and to insure their uniform distribution throughout the sheet as finally formed.

Preferably the alum Al (SO 181-1 0 is added after interspersion of the fibers and cork, but of course it may be added to the water prior to addition thereto of the fibers and cork. This water-soluble aluminum salt serves to sensitize the fibers and cork granules; and, while the theory is not fully understood, it is believed that the aluminum ions reduce the electrokinetic potential of the fibers and cork to substantially zero; thereby, the film of bound water which would interfere with proper adhesion of the binder to the fibers and cork is eliminated, and the binder may be uniformly and firmly attached to the fibers.

After the fibers have been sensitized, usually by agitation for a period of about 5 minutes, the sensitized fibers are treated with the sequestering agent, sodium citrate. This operation is performed by adding powdered sodium citrate to the slurry of sensitized fibers and agitating the mixture for about 4 minutes. The sodium citrate functions to sequester free aluminum ions which may be presout upon completion of the sensitizing step. Generally at least a slight excess of alum will be used to insure complete sensitizing, and it is this excess which is acted upon by the sequestering agent.

The alum and the sodium citrate may be added as powdered ingredients as mentioned above, or they each may be dissolved in water and added as solutions to the slurry.

The antioxidant which may be the chemical compound symmetrical di-beta-naphthyl-para-phenylenediamine sold under the trademark Agen'te White is next added to the slurry.

Following this, the carboxy methylcellulose is added to the mass in the tile chest. This material serves as a dispersing aid and assists in obtaining a uniform distribution of the binder over the fibers and cork granules.

In the next step of the process, the rubber latex is added to the tile chest and agitation is continued for about 3 minutes. The butadiene-styrene latex may contain about equal parts of each component, butadiene and styrene, in the copolymer.

After the latex has been dispersed in the slurry, the

alum-acetic acid precipitating solution is added. This mixture may be made up of 20 parts by weight of alum, 22.5 parts by weight of acetic acid, and 157.5 parts by weight of water, sufficient to get the alum and acetic acid into solution for proper addition to the slurry. The acetic acid acts as a mild sequestering agent, masking to a limited degree the action of the alum. The combination of ingredients elfects a uniform deposit of the binder on the fiber, minimizing any tendency for the binder to precipitate out in the water.

After addition of the alum-acetic acid precipitation solution, agitation of the slurry is continued, and in about 6 minutes the rubber latex is fully deposited in finely dispersed form and is uniformly distributed over and securely attached to the fibers. The slurry is essentially free of large flocculated lumps of rubber.

Thereafter, the stock so prepared is delivered to a conventional paper or felt-making machine, such as a Fourdrinier or other forming unit, and a sheet or other object is formed, water is removed therefrom, and if desired the product is compressed, as by calendering, and the binder coalesced to produce a product of the desired density, after which final drying may be accomplished.

Butadiene-styrene rubber latex (38% solids) (dry weight) 180.0 Dispersing agent (10% solids) (dry weight) 1.8 Water 72,0000

Alum-precipitant solution, q. s.

In the preparation of a felted product in accordance with this example, the wool fibers are suspended in the water in a stock chest or the like, and the slurry is agitated to maintain the fibers in suspension. The alum is then added as a powder to the slurry, and agitation is continued for about minutes. This effects sensitizing of the wool fibers in the same manner as the fibers and cork in Example I. The powdered sodium citrate is then added while agitation of the slurry is continued for about 4 minutes. The sequestering action which takes place is the same as in Example I.

The antioxidant which may be Agerite White as in Example I is next added to the slurry, followed by butadiene-styrene latex to which the dispersing agent has been added. This dispersing agent may be a sodium alkyl aryl sulphonate, sold under the trademark Tamol N. Agitation of the slurry with the added butadienestyrene latex is continued for about 3 minutes to uniformly disperse the latex throughout the slurry. Thereafter, a solution of alum in water is added to the slurry, the addition being continued until the latex has been fully deposited onto the fibers. A very finely dispersed and uniform deposit of the rubber is obtained, free of large flocculated lumps normally encountered with wool fibers.

The stock thus formed may be converted into a sheet on a paper machine, calendered, and dried in the same manner as Example I or may be otherwise formed into a product of the desired character.

Example III Parts by weight In the preparation of a product in accordance with this example, the same procedural steps as set forth in Example I may be followed. The quantity of Carboxy methylcellulose used in this example is somewhat greater than in Example I, and care should be used in its incorporation into the slurry.

Water 72,0000 Alum-acetic acid precipitant solution 200.0

The preparation of a product in accordance with this example may be elfected in the same manner as in Example I.

In the above examples, alum, aluminum sulphate, has been listed as the sensitizing agent for the fibers. It is preferred because of its low cost and ready availabliity, also because papermakers are familiar with the material and its properties since it is conventionally used as a sizing precipitant in paper manufacture. Other watersoluble aluminum salts may be substituted in Whole or in part for the alum, such as aluminum chloride, aluminum nitrate, or the like, supplying trivalent aluminum 10118.

The quantity of aluminum salt used to sensitize the fibers will vary with the type of fiber being treated, the quantity of water in the slurry, and other variable factors. There must be sufficient used to reduce the electrokinetic potential substantially to zero, as noted above. Where the fibers are to be dyed in the slurry, the quantity of alum should be sufiicient to properly mordant the fibers for dyeing and also sufficient to reduce the electrokinetic potential substantially to zero. A reasonable excess of the aluminum salt may be used but will require the addition of more of the sequesting agent. In Examples 1, III, and IV, 15 parts of alum are used with 600 parts of fibers or fibers and cork and about 9,000 gallons of water. This quantity is adequate to provide for the mordant dyeing of the fibers as well as reducing the zeta or electrokinetic potential of the fibers or fibers and cork substantially to zero, with a safe margin in excess to insure that proper sensitizing will be effected notwithstanding the variable factors which are normally encountered in factory production. In Example II, 7.5 parts of alum are used to sensitize the wool fibers. This has been found to be an adequate quantity, but as noted above may be increased without deleterious results. Since the sequestering agents are not inexpensive, it is desirable, of course, to keep the quantity of sensitizing agent used as low as commercially practicable, consistent with good industrial practice. A lower limit of about 7.5 parts of alum for 600 parts of fibers or other particles in 9,000 gallons of water is a safe lower limit for factory production.

The sequestering agent may be any water-soluble polycarboxylic acid, anhydride, or salt, or mixtures thereof. Examples of suitable acids are oxalic, maleic, succinic, glutaric, tartaric, citric, and ethylene diamine tetra-acetic. The salts may be any water-soluble salt of a polycarboxylic acid or mixture thereof such as the sodium, potassium, and lithium salts of the specific acids referred to above, or their equivalents. The term polycarboxylic acid as used herein embraces the anhydrides of the acids referred to.

Monocarboxylic acids and their salts, such as acetic acid, do not perform satisfactorily as sequestering agents in the treatment of the sensitized particles. Such monocarboxylic acids are ideally suited, however, as mild sequestering agents for controlling the activity of the precipitant solution to eifect a uniform deposit of the binder onto the sensitized and treated particles without substantial precipitation of the binder in the water.

The quantity of sequestering agent added to the slurry will depend upon the particular sensitizing agent selected and the quantity of trivalent aluminum ions to be sequestered. The sequestering agents are known to be effective in varying degrees depending upon the pH of the slurry to which the agents are added. It is preferred, of course, to keep the pH about neutral or above where an alkaline latex is to constitute the binder. Generally the quantity of sequestering agent will be in the ratio of .58 equivalent to 1.3 equivalents of sequestering agent for each equivalent of alum or other sensitizing agent. Preferably, a theoretical stoichiometric relationship between the sensitizing agent and the sequestering agent should be maintained, although a reasonable excess of sequestering agent may be employed, provided this excess is taken into account in the addition of the precipitation solution for the rubber latex or other binder.

Also where the white water is recirculated, sensitizing of the new batch of fibers will not be eifected until sufficient alum or other sensitizing agent has been added to properly act upon the fibers, the sequestering agent in the white water sequestering some of the aluminum ions from the added alum and reducing its sensitizing effect.

The binder precipitating solution may be any one of a number of products. The alum-acetic acid mixture is preferred. The substitutions referred to above which may be made for the alum in the sensitizing medium may be made for the alum in the precipitating mixture. Where the white water is to be recirculated, it is preferred to use a monocarboxylic acid, such as acetic acid, with the alum or other precipitating agent. If a polycarboxylic acid or salt is used as a part of the precipitating mixture, then when the white water is reused, the acid will tend to sequester aluminum ions from the sensitizing solution and will require the use of additional sensitizing material. The polycarboxylic acids and salts are efiicient sequestering agents; and since the precipitating action of the solution depends upon the presence of trivalent aluminum ions to act on the binder to be precipitated, excessive sequestering is undesirable. 0n the other hand, the monocarboxylic acids are less efiective sequestering agents, and they mask the effectiveness of the precipitant to a limited extent and insure good distribution of the binder on the filler particles without substantial precipitation of the binder in the water.

In the above examples, synthetic rubber latex and asphalt emulsion are recited as suitable binders. Other binders which are substantially insoluble in water may be substituted in whole or in part. Any of the synthetic rubbers, such as GR-S, GR-N, and neoprene, may be used. The various phenol-formaldehyde resins, and any of the resinous materials formed through unsaturated linkages such as the vinyl resins may be employed. Not only synthetic resinous materials but also natural resinous materials are contemplated, including coal tar byproducts and petroleum by-products as well as other thermosetting and thermoplastic resinous or rubbery material including natural rubber. The binder may be in the form of a dispersion of finely ground material as in the case of the artificial resinous materials, may be in emulsion form as in the asphalt emulsion of Example 1V, or may be in the form of latex as in Examples I, II, and III. Mixtures of various binder components may be used.

The quantity of binder added to the filler may be varied over a wide range. In some of the examples, 300 parts of binder have been incorporated with 600 parts of filler particles. In Example II, 180 parts of binder have been added to 600 parts of wool fibers. With the present process it is possible to add larger quantities of binder to difiicultly processable fillers such as unprocessed or lightly refined pulp fibers, wool, and the like.

While the invention is particularly useful in the preparation of products from wood fibers which are unprocessed or only lightly refined and which have a Canadian S-gram freeness of 570 or above and a permanganate number of 15 or above, the invention is not limited thereto but is applicable to other particulate materials, including cork, wood flour, all kinds of natural and synthetic fibers, clay, and the like or mixtures of two or more types or kinds ofparticles, such as products made from combinations of fiber and clay, or the fiber and cork products of some of the specific examples.

The process may be utilized in the preparation of many different products in addition to the sheet materials mentioned previously. The product may be molded into desired shapes, or may be formed into mats or blocks, as in the manufacture of cork composition products, suitable for severance into floats, gaskets, or the like. Where cork composition is to beproduced, the slurry of binder-coated cork granules prepared by the same procedural steps as set forth in the above examples will be drained, dried, and then charged into molds where the mass will be compressed and heat cured.

l claim:

Lln a method of uniformly depositing a water-insoluble binder onto filler particles suspended in an aqueous medium, the steps comprising first sensitizing the particles for the uniform deposit thereon of the binder by agitating the particles while dispersed in an aqueous medium having dissolved therein a water-soluble aluminum salt supplying trivalent aluminum ions in excess of those re ,quired for sensitizing of the particles, after said particles have been sensitized by the action of said trivalent aluminum ions in said first step adding to said slurry at least one compound of the group consisting of watersoluble polycarboxylic acids and water-soluble salts thereof and agitating the resulting slurry until substantially all said excess trivalent aluminum ions resulting from the first step have been removed from the slurry, then adding to said slurry a dispersion of water-insoluble binder, distributing said binder uniformly throughout said slurry, and depositing said binder as a uniform coating onto said sensitized and treated particles by thereafter adding to said slurry a precipitation agent for said binder and agitating said slurry.

2. The method of claim 1 in which .58 to about 1.3 equivalents of the sequestering agent are added for each equivalent of aluminum salt.

3. The method of claim 1 wherein the precipitation agent comprises a water-soluble aluminum salt supply-- ing trivalent aluminum ions and a water-soluble monocar'boxylic acid.

4. The method of claim 1 wherein the sensitizing agent is alum, the sequestering agent in the initial treatment of the sensitized fibers is a salt of citric acid, and the precipitation agent is alum and acetic acid.

5. The method of claim 1 in which there are 600 parts by weight of the particles to about 7.5 to 15 parts by weight of the water-soluble aluminum salt.

6. The method of claim 1 wherein the precipitation agent comprises a water-soluble aluminum. salt supplying trivalent aluminum ions and as a mild sequestering agent therefor at least one compound of the group consisting of water-soluble monocarboxylic acids and watersoluble salts thereof.

7. In a method of uniformly depositing a waterinsoluble binder onto filler particles suspended in an aqueous medium, the steps comprising first sensitizing the particles for the uniform deposit thereon. of the binder by agitating the particles while dispersed in an aqueous medium having dissolved therein a water-soluble aluminum salt supplying trivalent aluminum ions to reduce the electroltinetic potential of the fibers substantially to zero, after said particles have been so sensitized by the action of said trivalent aluminum ions in said first step adding to said slurry at least one compound of the group consisting of Water-soluble polycarboxylic acids and Watersoluble salts thereof and agitating the resulting slurry until substantially all excess trivalent aluminum ions resulting from the first step have been removed from the slurry, then adding a dispersion of water-insoluble binder to said slurry, distributing said binder uniformly throughout said slurry, and depositing said binder as a uniform coating onto said sensitized and treated particles by thereafter adding to said slurry a precipitation agent for said binder and agitating said slurry.

8. The method of claim 7 wherein the particles are fibrous and wherein the water-insoluble binder comprises a dispersion of rubber.

9. The method of claim 1 in which the particles comprise cellulose fibers having a minimum average permanganate number of 16 and a minimum average Canadian 3-gram freencss of 570.

10. The method of claim 9 wherein the fibers comprise lightly refined lcraft fibers.

11. In a method of making a synthetic rubber-bound fibrous product, the steps comprising first sensitizing the fibers for the uniform deposit thereon of a coating of synthetic rubber by agitating about 600 parts by Weight of said fibers While dispersed in an aqueous medium having dissolved therein about 7.5 to 15 parts by weight of aluminum sulphate Which supplies trivalent aluminum ions in excess of those required for sensitizing the fibers, after said fibers have been sensitized by the action of said trivalent aluminum ions from said aluminum sulphate in said first step adding to said slurry 1 part by Weight of sodium citrate for each 1 to 1 /2 parts by Weight of aluminum sulphate and agitating the resulting slurry until substantially all of said excess trivalent aluminum ions resulting from the first step have been removed from the slurry, then adding a dispersion of synthetic rubber to said slurry, distributing said synthetic rubber dispersion uniformly throughout said slurry, and depositing said synthetic rubber as a uniform coating onto said sensitized and treated fibers by thereafter adding to said slurry a precipitation agent for said synthetic rubber dispersion and agitating said slurry.

12. The method of claim 1 wherein the filler particles comprise cork granules.

13. The method of claim 12 wherein the slurry of binder-coated cork granules is drained, dried, and charged into a mold where the mass is compressed and heat-cured.

14. The method of claim 1 wherein the filler particles comprise Wool fibers.

15. In a method of uniformly depositing a dispersion of water-insoluble binder onto filler particles suspended in an aqueous slurry, the improvement which comprises incorporating in said slurry a precipitation agent comprising a water-soluble aluminum salt supplying trivalent aluminum ions and a water-soluble monocarboxylic acid and agitating said slurry.

References Cited in the file of this patent UNITED STATES PATENTS Feigley Oct. 7, 1952 Bixler Apr. 14, 1953 Feigley Dec. 4, 1956 OTHER REFERENCES Martell et al.: Chemistry of the Metal Chelate Compounds, Prentice-Hall, Inc., N. Y., 1952, pp. 2, 516, 534-535. 

1. IN A METHOD OF UNIFORMLY DEPOSITING A WATER-INSOLUBLE BINDER ONTO FILLER PARTICLES SUSPENDED IN AN AQUEOUS MEDIUM, THE STEPS COMPRISING FIRST SENSITIZING THE PARTICLES FOR THE UNIFORM DEPOSIT THEREON OF THE BINDER BY AGITATING THE PARTICLES WHILE DISPERSED IN AN AQUEOUS MEDIUM HAVING DISSOLVED THEREIN A WATER-SOLUBLE ALUMINUM SALT SUPPLYING TRIVALENT ALUMINUM IONS IN EXCESS OF THOSE REQUIRED FOR SENSITIZING OF THE PARTICLES, AFTER SAID PARTICLES HAVE BEEN SENSITIZED BY THE ACTION OF SAID TRIVALENT ALUMINUM IONS IN SAID FIRST STEP ADDING TO SAID SLURRY AT LEAST ONE COMPOUND OF THE GROUP CONSISTING OF WATERSOLUBLE POLYCARBOXYLIC ACIDS AND WATER-SOLUBLE SALTS THEREOF AND AGITATING THE RESULTING SLURRY UNTIL SUBSTANTIALLY ALL SAID EXCESS TRIVALENT ALUMINUM IONS RESULTING FROM THE FIRST STEP HAVE BEEN REMOVED FROM THE SLURRY, THEN ADDING TO SAID SLURRY A DISPERSION OF WATER-INSOLUBLE BINDER, DISTRIBUTING SAID BINDER UNIFORMLY THROUGHOUT SAID SLURRY, AND DEPOSITING SAID BINDER AS A UNIFORM COATING ONTO SAID SENSITIZED AND TREATED PARTICLES BY THEREAFTER ADDING TO SAID SLURRY A PRECIPITATION AGENT FOR SAID BINDER AND AGITATING SAID SLURRY. 